On Mon, Jul 10, 2017 at 05:17:27PM +1000, NeilBrown wrote:
> On Fri, Jul 07 2017, Phil Kauffman wrote:
>
> >> I can imagine /etc/mtab being read once for every line in /etc/exports,
> >> but unless your /etc/exports is very big, I can't easily see why it
> >> would be read 900 times.
> >> Maybe lots of different mount points are being accessed by something and
> >> each one triggers a few reads...
> >>
> >> Can you show your /etc/exports file?
> >
> > These get generated via puppet and dropped into /etc/exports.d/
> ...
> >
> > Here we can see that there are 960 files in /etc/exports.d
> > root@storage1:~# find /etc/exports.d/ -type f | wc
> > 960 960 88701
>
> Ahhh.. 960 exports. That could do it.
>
> $ grep -c crossmnt all_etc_exports.d_in_one_file.txt
> 957
>
> I get 957 - much the same number.
>
> $ grep open redacted_strace.txt | uniq -c
> 2 open("/var/lib/nfs/etab", O_RDONLY) = 6
> 942 open("/etc/mtab", O_RDONLY|O_CLOEXEC) = 6
> 1 open("/proc/net/rpc/nfsd.export/channel", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 6
>
> /etc/mtab was ready 942 for a single access. I would have expected 957,
> but maybe the system is dynamic and something changed between the two
> samples.
>
> This makes it fairly clear what is happening. Now we just need to fix
> it.
> One option would be to cache some of the details extracted from
> /etc/mtab, but that could get messy.
Would be possible to read /proc/self/mounts only once and use poll()
to detect changes?
The issue is not only the open()+read() in the process, but also
kernel has to compose the file from the mount tree. It's expensive.
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
http://karelzak.blogspot.com
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